Dynamic sub-carrier assignment in OFDM systems

ABSTRACT

A method and apparatus are provided for selecting and signaling the identity of sub-carriers to be used for transmission of data in a radio communication system, and for using other sub-carriers. A remote unit determines which sub-carriers are acceptable for use in data transmission by comparing the signal to interference ratio of each sub-carrier with a threshold. A base station transmits data over the acceptable sub-carriers at the optimum Link Mode or Link Modes. The base station may use some of the unacceptable sub-carriers for transmission of low sensitivity data at the optimum Link Mode, and may use some of the unacceptable sub-carriers for transmission of data at a lower Link Mode. The transmission power any unused unacceptable sub-carriers can be diverted to other sub-carriers.

FIELD OF THE INVENTION

This invention relates to digital radio communication systems employingmultiple sub-carriers, and more particularly to dynamic use ofsub-carriers within such systems.

BACKGROUND OF THE INVENTION

In a digital radio communication system a base station transmits asignal at a transmission rate to a remote unit through a radio channelhaving channel characteristics, such as an attenuation. The signal istransmitted using transmission parameters, such as a modulation leveland a coding rate. The transmission rate depends on the transmissionparameters. The transmission parameters are constrained by an acceptablebit error rate and by a signal to interference ratio of the signal, thelatter varying in time with the channel characteristics. Thecommunication system can use adaptive modulation to adjust thetransmission parameters to accommodate changes in channelcharacteristics over time. If a change in channel characteristicsresults in a lower signal to interference ratio, the modulation levelmust be reduced (for example, from 16-QAM to QPSK) or the coding ratemust be improved (for example, from ¾ to ⅔) in order to maintain theacceptable bit error rate, albeit at a lower transmission rate. If achange in channel characteristics results in a higher signal tointerference ratio, the base station can increase the modulation levelor decrease the coding rate in order to obtain a higher transmissionrate.

In a communication system that implements adaptive modulation, the basestation and the remote unit must be synchronized with respect to thetransmission parameters. In current communication systems the remoteunit determines a channel quality when the remote unit receives a frameof data. The remote unit may estimate, for example, the signal tointerference ratio of the channel. The remote unit sends a signal backto the base station reporting the channel quality. Using the channelquality report received from the remote unit, the base stationcalculates a set of optimum transmission parameters which the basestation will use in its next transmission of data. However, the basestation must first send the set of new optimum transmission parametersto the remote unit using the previous transmission parameters. Theremote unit receives the set of new optimum transmission parameters,interpreting the signal using the previous transmission parameters. Theremote unit then decodes subsequent frames of data using the new optimumtransmission parameters.

In communication systems that make use of multiple antennae fortransmission and reception, the transmission parameters may includeadaptive antenna and coding parameters. For example, some “smartantenna” systems may adaptively adjust their directional patternstowards the remote units. An outline of such systems may be found in thepaper by J. H. Winters, “Smart Antennas for Wireless Systems”, IEEEPers. Commun., vol. 5, no. 1, February 1998, pp. 23-27, which isincorporated by reference herein. Similarly, the radio system may makeuse of the multiple communication channels that exist betweentransmitters and receivers with multiple antennae. In this case, thetransmission parameters include both space (across multiple antennae)and time (different time of transmission) aspects that adapt thetransmissions to the multiple propagation environment. An outline ofsuch systems may be found in the paper by A. J. Paulraj and B. C. Ng,“Space-time Modems for Wireless Personal Communications”, IEEE Pers.Commun., vol. 5, no. 1, February 1998, pp. 36-48, which is incorporatedby reference herein.

In communication systems employing many sub-carriers, such as those thatemploy Orthogonal Frequency Division Multiplexing (OFDM systems), thechannel quality may vary with the frequency of each sub-carrier. OFDMsystems can use 1000 sub-carriers, and transmission of informationdescribing the channel quality and a set of optimum transmissionparameters for each sub-carrier would require significant overhead,reducing the efficiency of the communication system. In current methods,the signal to interference ratio is averaged over all sub-carriers sothat only one signal to interference ratio is reported to the basestation and only one set of new optimum transmission parameters istransmitted to the remote unit. In this method, the single set of newoptimum transmission parameters results in an unnecessarily lowtransmission rate for individual sub-carriers whose signal tointerference ratio is higher than the average signal to interferenceratio reported by the receiver.

SUMMARY OF THE INVENTION

The present invention provides a method of selecting and signalling theidentity of acceptable groups of sub-carriers in a radio communicationsystem. A remote unit receives a signal as more than one sub-carriersignal from a base station. The remote unit determines a channel quality(such as a signal to interference ratio or a reciprocal of an errorrate) of each group of sub-carrier signals, and compares the channelquality of each group of sub-carrier signals with a threshold. Asequence of numbers is generated, there being one number for each groupof sub-carrier signals. Each number has a value belonging to a first setof values if the channel quality of the corresponding group ofsub-carriers is above the threshold, and has a value belonging to asecond set of values if the channel quality of the corresponding groupof sub-carriers is not above the threshold, the two sets of valueshaving no values in common. The first set of values may consist of thevalue one and the second set of values may consist of the value zero, inwhich case each number in the sequence has a length of one bit. Theremote unit generates at least one value by which the base station candetermine one or more Link Modes, a Link Mode being a set oftransmission parameters. The remote unit transmits the sequence ofnumbers and the values by which the base station can determine the LinkMode or Link Modes.

The remote unit may calculate the average channel quality of groups ofsub-carriers whose channel quality is above the threshold, in which casethe average channel quality is transmitted to the base station. Theremote unit may also determine a Link Mode using the average channelquality, in which case the Link Mode is transmitted to the base station.The remote unit may alternatively determine a Link Mode for each groupof sub-carriers whose channel quality is above the threshold, in whichcase the sequence of numbers and the values by which the base stationcan determine the Link Mode of each sub-carrier whose channel quality isabove the threshold is transmitted. In the latter case, the sequence ofnumbers and the values by which the base station can determine the LinkModes can be combined into a single sequence of numbers.

The present invention also provides a method of assigning transmissiontasks to at least one sub-carrier in a radio communication system. Abase station receives a return signal, and extracts from the returnsignal a sequence of numbers, each number corresponding to one group ofsub-carriers, and at least one value by which the base station candetermine at least one Link Mode. The base station determines at leastone Link Mode based on the at least one value. The base station definesa set of acceptable groups of sub-carriers as all groups of sub-carriersfor which the corresponding number has a value belonging to a first setof values, and defines a set of unacceptable groups of sub-carriers asall groups of sub-carriers for which the corresponding number has avalue belonging to a second set of values, the two sets of values havingno values in common. The base station allocates for data transmission atone of the Link Modes the sub-carriers which belong to the groups ofsub-carriers within the set of acceptable groups of sub-carriers. In oneembodiment, the return signal includes an average channel quality andthe base station determines a single Link Mode based on the averagechannel quality. In another embodiment, the return signal includes areference to a Link Mode and the base station determines a single LinkMode based on the reference to the Link Mode. In yet another embodiment,the return signal includes references to one Link Mode for eachacceptable group of sub-carriers, possibly within the sequence ofnumbers, and the base station determines a Link Mode for each acceptablegroup of sub-carriers based on the corresponding reference. The basestation may allocate for low sensitivity data transmission sub-carrierswithin some of the unacceptable sub-carriers, may allocate for datatransmission at a low transmission rate sub-carriers within some of theremaining unacceptable sub-carriers, and may divert transmission powerfrom the remaining unused unacceptable sub-carriers to othersub-carriers.

The method provides improved efficiency of a communication system byallowing sub-carriers having a high signal to interference ratio to usea higher transmission rate. Sub-carriers having a low signal tointerference ratio can be used for less sensitive traffic, or theirtransmission power can be diverted to sub-carriers having a high signalto interference ratio.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying diagrams, in which:

FIG. 1 is a block diagram of a portion of a radio communication systemin which the invention is implemented;

FIG. 2 is a chart of example signal to interference ratios in severalsub-carriers;

FIG. 3 is a flow chart of a method by which a remote unit determines andconveys information concerning acceptable sub-carriers to a basestation;

FIG. 4 is a flow chart of a method by which a base station makes use ofinformation concerning acceptable sub-carriers;

FIG. 5 is a flow chart of an alternative method by which a remote unitdetermines and conveys information concerning acceptable sub-carriers toa base station; and

FIG. 6 is a flow chart of an alternative method by which a base stationmakes use of information concerning acceptable sub-carriers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a portion of a radio communication system is shown.The radio communication system employs a plurality of sub-carriers totransmit traffic from a base station 10 to a remote unit 16. Forexample, the radio communication system may employ Orthogonal FrequencyDivision Multiplexing. A signal generator 36 within the base station 10generates a signal 12. The signal is transmitted through a base stationtransmitting antenna 14. Each sub-carrier carries data encoded with aLink Mode. A Link Mode is a set of at least one transmission parameter,such as a modulation level, a coding rate, a symbol rate, a transmissionpower level, antenna directional parameters, or space-time codingparameters. The signal 12 propagates along a forward link to the remoteunit 16, where it is received at a remote unit receiving antenna 18 as areceived signal. A Fast Fourier Transform (FFT) processor 20 within theremote unit 16 separates the received signal into a plurality ofsub-carrier signals, there being one sub-carrier signal for eachsub-carrier. A decoder 22 within the remote unit decodes the sub-carriersignals to produce received data. Each sub-carrier signal is decodedusing a Link Mode appropriate to the sub-carrier of the sub-carriersignal. The decoded data is then passed to a user 24.

The sub-carrier signals are also passed to a sub-carrier analysisprocessor 26 within the remote unit. The sub-carrier analysis processor26 measures a signal to interference ratio (S/I) of each sub-carriersignal. The sub-carrier analysis processor 26 compares the S/I of eachsub-carrier signal with a threshold to determine which sub-carriers areacceptable sub-carriers. Acceptable sub-carriers are sub-carriers forwhich the measured S/I of the corresponding sub-carrier signal is higherthan the threshold. Sub-carriers for which the measured S/I of thecorresponding sub-carrier signal is not higher than the threshold areunacceptable sub-carriers. The sub-carrier analysis processor 26calculates an average S/I of the S/Is of the acceptable sub-carriers.Referring to FIG. 2, an example set of S/Is is shown. A horizontal axis40 indicates a sub-band number of each of twelve sub-carriers. Avertical axis 42 indicates the S/I in dB of the sub-carrier signals. Ifa threshold 44 having a value of 4 dB is used, then the sub-carrieranalysis processor 26 will identify eight of the sub-carriers (sub-bandnumbers 1, 2, 3, 4, 5, 10, 11, and 12) as being acceptable sub-carriers.The sub-carrier analysis processor 26 calculates the average S/I of theacceptable sub-carriers in FIG. 2 as having a value of 9.1 dB, whereasthe average S/I of all sub-carriers in FIG. 2 would be 6.8 dB.

Returning to FIG. 1, the remote unit 16 transmits a return signal 30along a reverse link to the base station 10 through a remote unittransmitting antenna 28, which may or may not be the same antenna as theremote unit receiving antenna 18. The return signal 30 includes theaverage S/I of acceptable sub-carriers and a sequence of numbersidentifying the acceptable sub-carriers. If a value of “1” is used toidentify acceptable sub-carriers and a value of “0” is used to identifyunacceptable sub-carriers, or the reverse, then the sequence of numberscan be a bitmask, using one bit to indicate the acceptability of eachsub-carrier. Of course other values can be used to indicate whichsub-carriers are acceptable and which sub-carriers are unacceptable, butthen more bits are required in the sequence of numbers for eachsub-carrier. In the example of FIG. 2, the remote unit 16 would transmitan average S/I having a value of 9.1 dB and a bitmask having a value of“111110000111”.

The return signal 30 is received by the base station 10 at a basestation receiving antenna 32, which may or may not be the same antennaas the base station transmitting antenna 14. An extractor 33 within thebase station extracts the average S/I from the return signal 30 andpasses it to a Link Mode evaluator 34 within the base station. Theextractor 33 also extracts the bitmask from the return signal 30 andpasses it to the signal generator 36. The extractor 33 is a processor.The Link Mode evaluator 34 selects an optimum Link Mode based on theaverage S/I received from the remote unit 16 and on requirements of thecommunication system, such as an acceptable bit error rate. The LinkMode evaluator 34 is a processor, and may be combined with the extractor33. The optimum Link Mode is passed to the signal generator 36 withinthe base station, along with the bitmask from the extractor 33 and alongwith data 38 which is to be transmitted.

The signal generator 36 encodes the data 38 using the optimum Link Mode,and generates a new signal. The signal generator 36 differentiates thesub-carriers into a set of acceptable sub-carriers and a set ofunacceptable sub-carriers. The new signal only codes the data 38 overthe acceptable sub-carriers. Whether a particular sub-carrier isacceptable or unacceptable is determined from the bitmask, or moregenerally from the sequence of numbers, most recently received from theremote unit 16. Each unacceptable sub-carrier can be allocated for oneof several uses. Zero or more of the unacceptable sub-carriers are usedto transmit low sensitivity traffic encoded using the optimum Link Mode.Low sensitivity traffic can include inessential but potentially usefulinformation, such as control bits or parity bits. Although unacceptablesub-carriers have a low S/I and may have a high bit error rate whenreceived by the remote unit, the low sensitivity of the traffictransmitted over these sub-carriers means that some errors aretolerable. Zero or more of the remaining unacceptable sub-carriers areused to transmit low rate traffic using a second Link Mode having alower transmission rate than the optimum Link Mode. Any remainingunacceptable sub-carriers are unused, and power that would otherwise beused to transmit information over the unused unacceptable sub-carriersis assigned to other sub-carriers.

Referring to FIG. 3, a method by which the remote unit 16 determines theacceptable sub-carriers and the average S/I is shown. The remote unitreceives a signal at step 80 at the remote unit receiving antenna 18. Atstep 82 the signal is separated into its sub-carrier signals by the FFTprocessor 20. The first sub-carrier is selected at step 84. At step 86the S/I of the sub-carrier signal of the selected sub-carrier ismeasured. If at step 88 the measured S/I is higher than a threshold,then the selected sub-carrier is an acceptable sub-carrier. At step 90 arunning total is increased by the measured S/I, a count of acceptablesub-carriers is increased by one, and the sequence of numbers,preferably a bitmask, is adjusted to identify the selected sub-carrieras being an acceptable sub-carrier. If at step 92 there are furthersub-carriers, then at step 94 the next sub-carrier is selected and thealgorithm returns to step 86 to measure the S/I of sub-carrier signal ofthe next sub-carrier. If at step 92 there are no further sub-carriers,then at step 96 the average S/I of acceptable sub-carriers is calculatedas the running total divided by the count of acceptable sub-carriers.Steps 82 to 96 are carried out by the sub-carrier analysis processor 26.At step 98 the average S/I and the sequence of numbers are transmittedto the base station through the remote unit transmitting antenna 28.Steps 84, 92, and 94 form a loop that cycles through each sub-carrier,although any method of measuring the S/I of each sub-carrier signalcould be used. Steps 90 and 96 calculate the average S/I, although anymethod of calculating the average S/I could be used. For example, thecount of the acceptable sub-carriers could be determined at step 96 fromthe sequence of numbers.

Referring to FIG. 4, a method by which the base station uses thesub-carriers is shown. At step 120 the base station receives a returnsignal at the base station receiving antenna 32. At step 122 thesequence of numbers, preferably a bitmask, and the average S/I areextracted from the return signal by the extractor 33. At step 124 theLink Mode evaluator 34 selects an optimum Link Mode based on the averageS/I. At step 126 the base station receives data to be transmitted. Thefirst sub-carrier is selected at step 128. At step 130 the base stationdetermines whether the selected sub-carrier is an acceptable sub-carrierby comparing the sub-band number of the sub-carrier with the sequence ofnumbers. For example, if the sequence of numbers is a bitmask with avalue of “111110001111” and the selected sub-carrier was the firstsub-carrier, then a comparison of the sub-band number and the bitmaskwould produce a value of “1” since that is the value of the first bit,and the selected sub-carrier would therefore be an acceptablesub-carrier. If the selected sub-carrier was the sixth sub-carrier, thena comparison of the sub-band number and the bitmask would produce avalue of “0” since that is the value of the sixth bit, and the selectedsub-carrier would therefore be an unacceptable sub-carrier. If at step130 the base station determines that the selected sub-carrier is anacceptable sub-carrier, then at step 132 the data is encoded fortransmission over the selected sub-carrier using the optimum Link Mode.If at step 130 the base station determines that the selected sub-carrieris an unacceptable sub-carrier, then at step 134 low sensitivity data isencoded for transmission over the selected sub-carrier using the optimumLink Mode. Alternatively, at step 134 regular data could be encoded fortransmission over the selected sub-carrier at a second Link Mode havinga lower transmission rate than the optimum Link Mode, or no data couldbe encoded for transmission over the selected sub-carrier and thetransmission power of the selected sub-carrier diverted to othersub-carriers. If at step 136 there are further sub-carriers, then atstep 138 the next sub-carrier is selected and the algorithm returns tostep 130 to determine whether the next sub-carrier is an acceptablesub-carrier. If at step 136 there are no further sub-carriers, then atstep 140 the signal is transmitted to the receiver. Steps 128 to 138 arecarried out by the signal generator 36. Steps 128, 136, and 138 form aloop that cycles through each sub-carrier, although any method ofdetermining which sub-carriers are acceptable sub-carriers could beused.

Signalling overhead can be reduced in a number of ways. Overhead on theforward link from the base station to the remote unit can be reduced ifthe remote unit calculates the optimum Link Mode itself, using analgorithm similar to that used by the Link Mode evaluator 34. After adelay sufficient to allow the return signal 30 to reach the base stationand to allow the signal 12 carrying a frame encoded using the optimumLink Mode to reach the remote unit, the decoder 22 decodes frames usingthe optimum Link Mode. The base station need not transmit the optimumLink Mode to the remote unit. If the frames include numbered packets,then synchronization of the optimum Link Mode can be achieved moreprecisely if the remote unit includes a packet number in the returnsignal rather than estimating the delay. The base station begins usingthe optimum Link Mode when it transmits a packet having the packetnumber identified in the return signal, and the remote unit begins usingthe optimum Link Mode when it receives a packet having the packet numberidentified in the return signal.

Overhead on the reverse link can be reduced if the remote unitcalculates the optimum Link Mode itself and transmits a reference to theoptimum Link Mode to the base station, as disclosed in a U.S. patentapplication entitled “Receiver based adaptive modulation scheme” byHashem et al., filed on Sep. 27, 2000, and assigned to the assignee ofthe present application, and incorporated by reference herein. Thereturn signal can be viewed more generally as including a sequence ofnumbers identifying acceptable sub-carriers, and a value by which thebase station can determine an optimum Link Mode. The value by which thebase station can determine an optimum Link Mode may be the average S/I,as described above. Alternatively, if the remote unit determines theoptimum Link Mode itself, then the value may be a reference to theoptimum Link Mode. If the optimum Link Mode is one of a set of allowedLink Modes agreed upon by the base station and the remote unit prior toa transmission, the reference may be an index to the optimum Link Modewithin the set of allowed Link Modes. The return signal 30 contains thesequence of numbers and the reference, but not the average S/I ofacceptable sub-carriers. The extractor 33 simply extracts the referenceto the optimum Link Mode from the return signal 30 at step 122, and theLink Mode evaluator 34 determines the optimum Link Mode at step 124 fromthe set of allowed Link Modes using the reference.

A different Link Mode can be used for each acceptable sub-carrier if theLink Modes are calculated by the remote unit and transmitted to the basestation. In this embodiment, the remote unit transmits more than onevalue by which the base station can determine more than one Link Mode,preferably as a sequence of references to sub-carrier Link Modes. Thesequence of numbers identifying the acceptable sub-carriers and thevalues by which the base station can determine the Link Modes can besent along the return channel separately, or can be combined by formingthe sequence of numbers from the sub-carrier Link Mode of eachacceptable sub-carrier and one or more distinct values identifyingunacceptable sub-carriers. If in the example of FIG. 2 a S/I of 10 dB orhigher (for example) allows use of a seventh Link Mode within the set ofallowed Link Modes, a S/I of 7 dB or higher allows use of a sixth LinkMode within the set of allowed Link Modes, and a S/I of 4 dB or higherallows use of a fifth Link Mode within the set of allowed Link Modes (ahigher ordinal rank of Link Mode having a higher transmission rate),then the remote unit transmits a sequence of numbers having a value of“777660000567” to the base station along the reverse link. A numberhaving a value of “0” indicates that the corresponding sub-carrier isunacceptable, and a number having a value other than zero indicates boththat the corresponding sub-carrier is acceptable and the Link Mode to beused when encoding data for transmission over that sub-carrier. Themethod carried out by the remote unit is shown in FIG. 5. The methodshown in FIG. 5 is similar to the method shown in FIG. 3, except thatthe average S/I is not calculated (eliminating steps 90 and 96), asub-carrier Link Mode is evaluated for each sub-carrier (adding steps 89and 91), and the average S/I is not included in the return signal(replacing step 98 with step 99). At step 89 a Link Mode evaluator (notshown in FIG. 1, and which may or may not be a component of thesub-carrier analysis processor) in the remote unit determines the LinkMode for the selected sub-carrier based on the S/I of the selectedsub-carrier measured at step 86, as disclosed in patent applicationentitled “Receiver based adaptive modulation scheme”. At step 91 thesequence of references to sub-carrier Link Modes is adjusted by settingthe corresponding number in the sequence of numbers to be a reference tothe Link Mode determined at step 89.

The method carried out by the base station in the embodiment in whichthe Link Mode of each acceptable sub-carrier is transmitted to the basestation is shown in FIG. 6. The method shown in FIG. 6 is similar to themethod shown in FIG. 4, except that the average S/I is not included inthe return signal (replacing step 122 with step 123), and the step ofselecting the Link Mode (formerly step 124) is now contained within theloop that cycles through the sub-carriers, at step 131. Unacceptablesub-carriers are identified at step 130 by numbers having a value of “0”within the sequence of numbers. Acceptable sub-carriers are identifiedat step 130 by numbers having a value other than “0”, and the signalgenerator 36 polls the Link Mode evaluator 34 for the Link Mode of suchsub-carriers by passing the value of the number to the Link Modeevaluator 34. As there is no single Link Mode, low sensitivity data canbe encoded over unacceptable sub-carriers at step 134 using a Link Modehaving a low transmission rate, or the transmission power from theunacceptable sub-carriers can be diverted to other sub-carriers.

Overhead can be further reduced if the sub-carriers are grouped intogroups of sub-carriers by the components of the remote unit and the basestation. Sub-carriers having carrier frequencies close to each otherwill usually have similar channel qualities, so determining whether anentire group of sub-carriers is an acceptable group or an unacceptablegroup will offer a good approximation to correctly defining theindividual sub-carriers as acceptable or unacceptable. Overhead is savedbecause fewer numbers are needed in the sequence of numbers definingacceptable and unacceptable sub-carriers. The base station assigns thesame transmission task for every sub-carrier within a group ofsub-carriers, as the sequence of numbers identifying acceptable groupsof sub-carriers contains one number for each group of sub-carriers.Furthermore, in the embodiment in which the remote unit calculates morethan one Link Mode, the remote unit can calculate a group Link Mode foreach acceptable group of sub-carriers. In general, there will be fewergroup Link Modes to calculate and transmit to the base station thanthere would be individual sub-carrier Link Modes. The base station usesthe same group Link Mode for every sub-carrier within an acceptablegroup of sub-carriers. Of course each group of sub-carriers may containonly one sub-carrier, as in the various embodiments described above.

The various embodiments have been described using a S/I as a measure ofchannel quality. Other measures of channel quality could be used and anaverage channel quality reported to the base station, as long as thechannel quality is such that an optimum Link Mode can be determined. Forexample, the sub-carrier analysis processor may assess the channelquality of each sub-carrier by observing an error rate of the receiveddata. The sub-carrier analysis processor 26 may determine the error ratefrom the decoder 22 from the error correcting codes. An error ratethreshold can be set (similarly to the S/I threshold discussed above).Acceptable sub-carriers would be those sub-carriers with an error ratelower than the threshold (that is, with fewer errors), and unacceptablesub-carriers would be those sub-carriers with an error rate higher thanthe threshold (that is, with more errors). The sub-carrier analysisprocessor 26 would then calculate the average error rate of theacceptable sub-carriers, which would then be transmitted to the basestation in the same manner as the average signal to interference ratio,as described above. A logical equivalent to the relationships betweenthe error rate and the threshold is achieved if the sub-carrier analysisprocessor compares the reciprocal of the error rate with a threshold. Insuch a case, acceptable sub-carriers are those for which the reciprocalof the error rate is above the threshold and unacceptable sub-carriersare those for which the reciprocal of the error rate is below thethreshold. This has the advantage of preserving the conditionalrelationship between the channel quality and the threshold which is usedwhen the channel quality is a signal to interference ratio. In eithercase, the average error rate of acceptable sub-carriers or thereciprocal of the average error rate of acceptable sub-carriers can betransmitted to the base station for use by the base station indetermining an optimum Link Mode.

Although the invention has been developed for use with systems whichemploy Orthogonal Frequency Division Multiplexing, it is possible thatthe invention could be used with other systems which employ multiplesub-carriers simultaneously, and the invention is therefore considerednot to be limited to OFDM systems.

What has been described is merely illustrative of the application of theprinciples of the invention. Other arrangements and methods can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention. Each processor describedabove, in particular the sub-carrier analysis processor, the extractor,and the Link Mode evaluator, may be any computing apparatus containinglogic for executing the described functionality. For example, any of theprocessors may be a single processor, more than one processor, or acomponent of a larger processor. The logic may comprise externalinstructions or internal circuitry.

We claim:
 1. A method by a remote unit in a radio communication systemwhich employs Orthogonal Frequency Division Multiplexing, the remoteunit receiving a signal as a plurality of sub-carrier signals from abase station, each sub-carrier signal belonging to one of a plurality ofgroups of sub-carrier signals, the method comprising the steps of:determining a channel quality of each group of sub-carrier signals;comparing the channel quality of each group of sub-carrier signals witha threshold; generating a sequence of numbers, there being one numberfor each group of sub-carrier signals, each number having a valuebelonging to a first set of values if the channel quality of thecorresponding group of sub-carrier signals is above the threshold andhaving a value belonging to a second set of values if the channelquality of the corresponding group of sub-carrier signals is not abovethe threshold, the second set of values having no values in common withthe first set of values; generating at least one value by which the basestation can determine at least one Link Mode, a Link Mode being a set ofat least one transmission parameter; and transmitting the sequence ofnumbers and the at least one value by which the base station candetermine at least one Link Mode.
 2. The method of claim 1 comprisingthe further step of calculating an average channel quality of groups ofsub-carrier signals whose channel quality is above the threshold,wherein the at least one value by which the base station can determineat least one Link Mode is the average channel quality, wherein the firstset of values consists of the value one and the second set of valuesconsists of the value zero, and wherein each number in the sequence ofnumbers has a length of one bit.
 3. The method of claim 1 comprising thefurther steps of: calculating an average channel quality of groups ofsub-carrier signals whose channel quality is above the threshold; anddetermining an optimum Link Mode based on the average channel quality;and wherein the at least one value by which the base station candetermine at least one Link Mode is the optimum Link Mode, wherein thefirst set of values consists of the value one and the second set ofvalues consists of the value zero, and wherein each number in thesequence of numbers has a length of one bit.
 4. The method of claim 1comprising the further step of determining a group Link Mode for eachgroup of sub-carrier signals having a channel quality above thethreshold, wherein the at least one value by which the base station candetermine at least one Link Mode comprises a sequence of references tothe group Link Modes, wherein the second set of values consists of thevalue zero, and wherein the sequence of numbers and the sequence ofreferences are combined, the first set of values thereby comprisingreferences to group Link Modes.
 5. A remote unit for a radiocommunication system which employs Orthogonal Frequency DivisionMultiplexing, the remote unit receiving a plurality of sub-carriersignals from a base station, each sub-carrier signal belonging to one ofa plurality of groups of sub-carrier signals, the remote unitcomprising: means for determining a channel quality of each group ofsub-carrier signals; means for comparing the channel quality of eachgroup of sub-carrier signals with a threshold; means for generating asequence of numbers, there being one number for each group ofsub-carrier signals, each number having a value belonging to a first setof values if the channel quality of the corresponding group ofsub-carrier signals is above the threshold and having a value belongingto a second set of values if the channel quality of the correspondinggroup of sub-carrier signals is not above the threshold, the second setof values having no values in common with the first set of values; meansfor generating at least one value by which the base station candetermine at least one Link Mode, a Link Mode being a set of at leastone transmission parameter; and means for transmitting the sequence ofnumbers and the at least one value by which the base station candetermine at least one Link Mode.
 6. The remote unit of claim 5 furthercomprising means for calculating an average channel quality of groups ofsub-carrier signals whose channel quality is above the threshold, andwherein the at least one value by which the base station can determineat least one Link Mode is the average channel quality.
 7. The remoteunit of claim 6 wherein the channel quality of each group of sub-carriersignals is an average signal to interference ratio (S/I) of thesub-carrier signals in the group.
 8. The remote unit of claim 6 whereinthe channel quality of each group of sub-carrier signals is a reciprocalof an average of an error rate of each sub-carrier signal in the group.9. The remote unit of claim 6 wherein the first set of values consistsof the value one and the second set of values consists of the valuezero, and each number in the sequence of numbers has a length of onebit.
 10. The remote unit of claim 7 wherein the first set of valuesconsists of the value one and the second set of values consists of thevalue zero, and each number in the sequence of numbers has a length ofone bit.
 11. The remote unit of claim 5 further comprising: means forcalculating an average channel quality of groups of sub-carrier signalswhose channel quality is above the threshold; and means for determiningan optimum Link Mode based on the average channel quality; and whereinthe at least one value by which the base station can determine at leastone Link Mode is the optimum Link Mode.
 12. The remote unit of claim 11wherein the channel quality of each group of sub-carrier signals is anaverage signal to interference ratio (S/I) of the sub-carrier signals inthe group.
 13. The remote unit of claim 11 wherein the first set ofvalues consists of the value one and the second set of values consistsof the value zero, and each number in the sequence of numbers has alength of one bit.
 14. The remote unit of claim 12 wherein the first setof values consists of the value one and the second set of valuesconsists of the value zero, and each number in the sequence of numbershas a length of one bit.
 15. The remote unit of claim 5 furthercomprising means for determining a group Link Mode for each group ofsub-carrier signals having a channel quality above the threshold, andwherein the at least one value by which the base station can determineat least one Link Mode comprises a sequence of references to the groupLink Modes.
 16. The remote unit of claim 15 wherein the channel qualityof each group of sub-carrier signals is an average signal tointerference ratio of the sub-carrier signals in the group.
 17. Theremote unit of claim 15 wherein the sequence of numbers and the sequenceof references are combined, the first set of values thereby comprisingreferences to group Link Modes, and the second set of values consists ofthe value zero.
 18. The remote unit of claim 17 wherein the channelquality of each group of sub-carrier signals is an average signal tointerference ratio of the sub-carrier signals in the group.
 19. A remoteunit for a radio communication system which employs a plurality ofsub-carriers, the remote unit receiving a plurality of sub-carriersignals from a base station, each sub-carrier signal belonging to one ofa plurality of groups of sub-carrier signals, the remote unitcomprising: means for determining a channel quality of each group ofsub-carrier signals; means for comparing the channel quality of eachgroup of sub-carrier signals with a threshold; means for generating asequence of numbers, there being one number for each group ofsub-carrier signals, each number having a value belonging to a first setof values if the channel quality of the corresponding group ofsub-carrier signals is above the threshold and having a value belongingto a second set of values if the channel quality of the correspondinggroup of sub-carrier signals is not above the threshold, the second setof values having no values in common with the first set of values; meansfor generating at least one value by which the base station candetermine at least one Link Mode, a Link Mode being a set of at leastone transmission parameter; and means for transmitting the sequence ofnumbers and the at least one value by which the base station candetermine at least one Link Mode.
 20. A processor in a remote unit for aradio communication system which employs Orthogonal Frequency DivisionMultiplexing, the remote unit receiving a plurality of sub-carriersignals from a base station, each sub-carrier signal belonging to one ofa plurality of groups of sub-carrier signals, the processor includinginstructions for: determining a channel quality of each group ofsub-carrier signals; comparing the channel quality of each group ofsub-carrier signals with a threshold; generating a sequence of numbers,there being one number for each group of sub-carrier signals, eachnumber having a value belonging to a first set of values if the channelquality of the corresponding group of sub-carrier signals is above thethreshold and having a value belonging to a second set of values if thechannel quality of the corresponding group of sub-carrier signals is notabove the threshold, the second set of values having no values in commonwith the first set of values; and generating at least one value by whichthe base station can determine at least one Link Mode, a Link Mode beinga set of at least one transmission parameter.
 21. The processor of claim20 further comprising instructions for calculating an average channelquality of groups of sub-carrier signals whose channel quality is abovethe threshold, and wherein the at least one value by which the basestation can determine at least one Link Mode is the average channelquality, wherein the first set of values consists of the value one andthe second set of values consists of the value zero, and wherein eachnumber in the sequence of numbers has a length of one bit.
 22. Theprocessor of claim 20 further comprising instructions for: calculatingan average channel quality of groups of sub-carrier signals whosechannel quality is above the threshold; and determining an optimum LinkMode from the average channel quality; and wherein the at least onevalue by which the base station can determine at least one Link Mode isa reference to the optimum Link Mode, wherein the first set of valuesconsists of the value one and the second set of values consists of thevalue zero, and wherein each number in the sequence of numbers has alength of one bit.
 23. The processor of claim 20 further comprisinginstructions for determining a group Link Mode for each group ofsub-carrier signals whose channel quality is higher than the threshold,wherein the at least one value by which the base station can determineat least one Link Mode comprises a sequence of references to the groupLink Modes, wherein the second set of values consists of the value zero,and wherein the sequence of numbers and the sequence of references arecombined, the first set of values thereby comprising references to groupLink Modes.